About 12 billion years ago, a dwarf galaxy known as LKH collided with a young Milky Way and merged with it. This artist’s concept portrays that collision. NASA’s Hubble Space Telescope uncovered definitive evidence of this collision by studying globular star clusters.

Source: https://www.nasa.gov/image-detail/stsci-01kz4n4d8g9ak26cf28hd5nt5n-d2d764/